mus drag and drop: return the completed effect to the caller.

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index 6d4782528b725a5ea7beb1088809b7f37f3846d0..12bad8ff2e31d5a2f520a79def383f6668e9a424 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -452,8 +452,12 @@ interface WindowTreeClient {
                  gfx.mojom.Point position,
                  uint32 effect_bitmask) => (uint32 effect_taken);
 
-  // Called after OnDragDrop completes for every connection which received an
-  // OnDragDropStart() message. This signals that a client can forget the
+  // Called on the client that requested PerformDragDrop() to return which drag
+  // action was completed. This is called instead of OnChangeCompleted().
+  OnDragCompletedReturnValue(uint32 window, bool success, uint32 effect_taken);

[sky, 2016/09/20 22:20:26]

Please name this to match the call that initiated it. Perhaps
OnDragDropCompleted or perhaps OnPerformDragDropCompleted. I wouldn't bother
with the 'ReturnValue' part of it. That's implied by the args.

+
+  // Called after OnCompleteDrop completes for every connection which received
+  // an OnDragDropStart() message. This signals that a client can forget the
   // |drag_data| passed in via the first message.
   OnDragDropDone();